1. Field of the Invention
This invention relates to a cathode for use in an electrolytic cell, and in particular to a cathode which has a low hydrogen over-voltage when used in the electrolysis of water or aqueous solutions, for example aqueous alkali metal chloride solutions.
2. Background of Related Art
The voltage at which a solution may be electrolyzed at a given current density is made up of and is influenced by a number of features, namely the theoretical electroyzing voltage, the over-voltages at the anode and cathode, the resistance of the solution which is electrolyzed, the resistance of the diaphragm or membrane, if any, positioned between the anode and cathode, and the resistance of the metallic conductors and their contact resistances.
As the cost of electrolysis is proportional to the voltage at which electrolysis is effected, and in view of the high cost of electrical power, it is desirable to reduce the voltage at which a solution is electrolyzed to as low as a value as possible. In the electrolysis of water or aqueous solutions--, --; there is considerable scope for achieving such a reduction in the electrolyzing voltage by reducing the hydrogen over-voltage at the cathode.
There have been many prior proposals of means of achieving such a reduction in hydrogen over-voltage.
For example, it is known that the hydrogen over-voltage at a cathode may be reduced by increasing the surface area of the cathode, for example by etching the surface of the cathode in an acid, or by grit-blasting the surface of the cathode, or by coating the surface of the cathode with mixture of metals, for example a mixture of nickel and aluminium, and selectively leaching one of the metals, for example aluminium, from the coating.
Other methods of achieving a low hydrogen over-voltage cathode which have been described involve coating the surface of a cathode with an electrocatalytically-active material which comprises a platinum group metal and/or an oxide thereof. Examples of such prior disclosures include the following.
U.S. Pat. No. 4,100,049 discloses a cathode comprising a substrate of iron, nickel, cobalt or alloys thereof and a coating of a mixture of a precious metal oxide, particularly palladium oxide, and a valve metal oxide particularly zirconium oxide.
British Patent 1511719 discloses a cathode comprising a metal substrate, which may be ferrous metal, copper or nickel, a coating of cobalt, and a further coating consisting of ruthenium.
Japanese Patent Publication 54090080 discloses pre-treating an iron cathode with perchloric acid followed by sinter coating the cathode with cathode active substances which may be ruthenium, iridium, iron or nickel in the form of the metal or a compound of the metal.
Japanese Patent Publication 54110983 discloses a cathode, which may be of mild steel, nickel or nickel alloy, and a coating of a dispersion of nickel or nickel alloy particles and a cathode activator which comprises one or more of platinum, ruthenium, iridium, rhodium, palladium or osmium metal or oxide.
Japanese Patent Publication 53010036 discloses a cathode having a base of a valve metal and a coating of an alloy of at least one platinum group metal and a valve metal, and optionally a top coating of at least one platinum group metal.
European Patent 0 129 374 describes a cathode which comprise a metallic substrate and a coating having at least an outer layer of a mixture of at least one platinum group metal and at least one platinum group metal oxide in which the platinum group metal in the mixture with the platinum group metal oxide comprises from 2% to 30% by weight of the mixture.